This invention relates to a vacuum cooling method in which food or the like is cooled by decreasing pressure, and to an apparatus for practicing the method (hereinafter referred to as a vacuum cooling apparatus, when applicable).
A vacuum cooling apparatus is used generally for cooling heated and processed foodstuffs. Foodstuffs to be cooled are put in a container. Under this condition, when the container is evacuated, the water in the foodstuffs boils because its boiling point is decreased by the evacuation of the container. The boiling of the water removes the heat in the foodstuffs, thus decreasing the temperature of the foodstuffs. This cooling action also occurs inside the foodstuffs. Therefore, the vacuum cooling apparatus cools food more quickly than a ventilation type cooling apparatus, for example. Further, the vacuum cooling apparatus is very sanitary because air goes into the container only when the container is opened to release the vacuum.
Previously, the vacuum cooling apparatus has been operated in a batchwise fashion. That is, foodstuffs are put in a container, and the container is evacuated to a predetermined degree. After the foodstuffs are cooled as required, the container is opened and the foodstuffs are removed.
The above-described vacuum cooling apparatus has the following disadvantage. With the vacuum cooling apparatus, it is impossible to process foodstuffs successively without wasting time. For instance, when providing frozen vegetables, the vegetables first are subjected to pretreatment such as washing, and then are peeled and cut, and finally are cooked. The cooked vegetables are cooled, frozen, and packed. In order to cool the foodstuffs, the batchwise vacuum cooling apparatus is used, so that the food manufacturing operation is suspended from time to time. That is, it is necessary to hold the foodstuffs temporarily until they are processed with the vacuum cooling apparatus. In this case the waiting time is variable. The foodstuffs cooked earlier have a longer waiting time before they are cooled with the apparatus, so that they are held at high temperature for a long time. Extended high temperatures are not good for sanitation. In addition, because of the variable waiting time, the processed foodstuffs are variable in quality.